I have a 29 gallon and am looking for one or two larger, mellow fish to add in with my 6 danios (3 zebra, 3 neon yellow), 1 pearl angelfish, and 1 clown loach. I know, eventually, I will need a 125gl for my clown loach, but they grow VERY slow so I have some time before she outgrows the tank, and before her and the angel give eachother grief (if they will). I don't want to overcrowd the tank but I do want larger more mellow fish as the danios and clown loach are insanely active. I also plan on getting an algae eater and am wondering which is best as they tend to grow very large, unless I get a couple of dwarfs. I know they loaches eat snails, but I don't plan on feeding him more than one at a time and may have to crack the shells at first...YIKES!

I'm not sure about adding any more large fish to a 29g that already houses an angelfish and the loach.

As you probably already know, clown loaches enjoy the company of other clown loaches. Now, you've already pointed out that a 29g tank is far too small for clowns so I'd refrain from buying more of them.

Instead, you could get a small herd of corydoras catfish for the bottom levels. Depending on the species, some can be more active than others but often they're pretty active little guys but they should stay on the bottom and not really do much in terms of stressing the angelfish. Furthermore, clown loaches are known to school with corydoras, so this situation might alleviate some of the stress the loach will have due to living on his own.

And yes, clown loaches are slow growers but do what you can to prevent his growth from getting stunted! He should really be moved to a larger tank ASAP but I would say as soon as he breaks the 5-6" mark he really needs more room (and more loach buddies).

yeah keep in mind you have a cichlid in that 29 gal. angels can get very aggressive when it comes to territory. and the clown loach will get big in size also. so really in my opinion your space in the tank is filled.